6.3. The apt-cache Command

6.3. The `apt-cache` Command

The apt-cache command can display much of the information stored in APT's internal database. This information is a sort of cache since it is gathered from the different sources listed in the sources.list file. This happens during the apt update operation.

VOCABULARY Cache

A cache is a temporary storage system used to speed up frequent data access when the usual access method is expensive (performance-wise). This concept can be applied in numerous situations and at different scales, from the core of microprocessors up to high-end storage systems.

In the case of APT, the reference Packages files are those located on Debian mirrors. That said, it would be very ineffective to go through the network for every search that we might want to do in the database of available packages. That is why APT stores a copy of those files (in /var/lib/apt/lists/) and searches are done within those local files. Similarly, /var/cache/apt/archives/ contains a cache of already downloaded packages (see sidebar GOING FURTHER The cache of .deb files) to avoid downloading them again if you need to reinstall them after a removal.

On the other hand, it is mandatory to run apt update regularly to update the cache. Otherwise your package search results will always miss the latest updates distributed by the Debian mirrors.

The apt-cache command can do keyword-based package searches with apt-cache search keyword. It can also display the headers of the package's available versions with apt-cache show package. This command provides the package's description, its dependencies, the name of its maintainer, etc. Note that apt search, apt show, aptitude search, and aptitude show work in the same way.

ALTERNATIVE axi-cache

apt-cache search is a very rudimentary tool, basically implementing grep on package's descriptions. It often returns too many results or none at all when you include too many keywords.

axi-cache search term, on the other hand, provides better results, sorted by relevancy. It uses the Xapian search engine and is part of the apt-xapian-index package which indexes all package information (and more, like the .desktop files from all Debian packages). It knows about tags (see sidebar GOING FURTHER The Tag field) and returns results in a matter of milliseconds.

$ axi-cache search package use::searching
axi-cache search package use::searching
30 results found.
Results 1-20:
100% packagesearch - GUI for searching packages and viewing package information
94% mlocate - transitional dummy package
87% cruft - transitional package
86% debtags - Debian Package Tags support tools
85% whohas - query multiple distributions' package archives
84% recoll - Personal full text search package
81% apt-forktracer - utility for tracking non-official package versions
[..]
68% wajig - unified package management front-end for Debian/Ubuntu
More terms: debian dctrl packagesearch information aptitude manager dpkg
More tags: admin::package-management works-with::software:package role::program suite::debian interface::commandline scope::application scope::utility
`axi-cache more' will give more results

Some features are more rarely used. For instance, apt-cache dumpavail displays the headers of all available versions of all packages. apt-cache pkgnames displays the list of all the packages which appear at least once in the cache.

One of the features which might come in handy more often is apt-cache policy, described in the following section.

6.3.1. The `apt-cache policy` Command

The apt-cache policy command displays the pinning priorities and distribution properties of each package source as explained in Section 6.2.5, “Managing Package Priorities”. It can also show the pinning priorities for all available versions and sources of a package. For the sources.list example used in Example 6.2, “/etc/apt/sources.list file for users of Debian Stable” and APT::Default-Release set to "/^bookworm(|-security|-updates)$/", the output will look like this:

$ apt-cache policy
Package files:
 100 /var/lib/dpkg/status
     release a=now
 990 http://deb.debian.org/debian bookworm-updates/non-free-firmware amd64 Packages
     release v=12-updates,o=Debian,a=stable-updates,n=bookworm-updates,l=Debian,c=non-free-firmware,b=amd64
     origin deb.debian.org
 990 http://deb.debian.org/debian bookworm-updates/main amd64 Packages
     release v=12-updates,o=Debian,a=stable-updates,n=bookworm-updates,l=Debian,c=main,b=amd64
     origin deb.debian.org
 990 http://security.debian.org/debian-security bookworm-security/non-free-firmware amd64 Packages
     release v=12,o=Debian,a=stable-security,n=bookworm-security,l=Debian-Security,c=non-free-firmware,b=amd64
     origin security.debian.org
 990 http://security.debian.org/debian-security bookworm-security/main amd64 Packages
     release v=12,o=Debian,a=stable-security,n=bookworm-security,l=Debian-Security,c=main,b=amd64
     origin security.debian.org
 990 http://deb.debian.org/debian bookworm/non-free-firmware amd64 Packages
     release v=12.5,o=Debian,a=stable,n=bookworm,l=Debian,c=non-free-firmware,b=amd64
     origin deb.debian.org
 990 http://deb.debian.org/debian bookworm/main amd64 Packages
     release v=12.5,o=Debian,a=stable,n=bookworm,l=Debian,c=main,b=amd64
     origin deb.debian.org
Pinned packages:

apt-cache policy can also show the pinning priorities for all available versions and sources of a given package.

$ apt-cache policy adb
adb:
  Installed: 1:29.0.6-28
  Candidate: 1:29.0.6-28
  Version table:
     1:33.0.3-2~bpo12+1 100
        100 http://deb.debian.org/debian bookworm-backports/main amd64 Packages
 *** 1:29.0.6-28 990
        990 http://deb.debian.org/debian bookworm/main amd64 Packages
        100 /var/lib/dpkg/status

Although there is a newer version of adb in the bookworm-backports repository, APT will not install it automatically based on the priority. One would have to use apt install adb/bookworm-backports or add a higher pinning priority to /etc/apt/preferences.d/adb.pref:

Package: adb
Pin: release o=Debian Backports,n=bookworm-backports
Pin-Priority: 1001

$ apt-cache policy limnoria
adb:
  Installed: 1:29.0.6-28
  Candidate: 1:33.0.3-2~bpo12+1
  Version table:
     1:33.0.3-2~bpo12+1 1001
        100 http://deb.debian.org/debian bookworm-backports/main amd64 Packages
 *** 1:29.0.6-28 990
        990 http://deb.debian.org/debian bookworm/main amd64 Packages
        100 /var/lib/dpkg/status